Before setting up your custom domain with your DNS provider you need to add a custom domain to your GitHub Pages site on GitHub.
On GitHub, navigate to your GitHub Pages site's repository.
Under your repository name, click Settings.
Under "Custom domain", add or remove your custom domain and click Save.
Tip: To check if your custom domain is supported, see "About supported custom domains."
Set up your custom domain with your DNS provider. For an overview of the next steps, see "Quick start: Setting up a custom domain."